Fabrication of a film bulk acoustic wave resonator from nano-crystalline diamond

Diamond & Related Materials 18 (2009) 253–257

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Diamond & Related Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d i a m o n d

Fabrication of a film bulk acoustic wave resonator from nano-crystalline diamond S. Shikata a,⁎, S. Fujii b, T. Sharda c a b c

Diamond Research Center, National Institute of Advanced Industrial Science and Technology (AIST), AIST TC2-13, 1-1-1 Umezono, Tsukuba 305-8568, Japan Seiko-Epson Corp., 1010 Fujimi, Fujimi-machi, Suwa-gun 339-0295, Japan Seki Corporation, 5-6-30 Kiba, Koto-ku, Tokyo 135-0042, Japan

a r t i c l e

i n f o

Available online 9 September 2008 Keywords: Acoustic FBAR Nano-crystalline diamond (NCD) Frequency device

a b s t r a c t A proposed new type of diamond-based frequency device has been fabricated for the first time through the use of nano-crystalline diamond as the thin film component of a Film Bulk Acoustic Resonator (FBAR) device. The new fabrication process uses Si MEMS-based fabrication techniques on 200 nm thick nano-crystalline diamond deposited on a 4 inch silicon wafer. After completely removing the silicon substrate below the device by the BOSCH process, one port resonators with a diaphragm structure have been fabricated. All the devices produced in the first fabrication run operated successfully with a high frequency response of 3.5 GHz. © 2008 Elsevier B.V. All rights reserved.

1. Introduction For acoustic wave device applications, surface acoustic wave (SAW) devices based on poly-crystalline diamond, combined with various kind of piezo-electric thin films such as ZnO, AlN, LiNbO3, LiTaO3 and KNbO3 and additional SiO2 for temperature compensation, have been investigated and several material systems have been proposed [1–7]. Among them, SiO2 / ZnO/ diamond structure SAW devices have been successfully developed and excellent SAW characteristics observed for the Sezawa mode wave: a phase velocity of 10,000 m/s, an electro-mechanical coupling coefficient (K2) of 1.2% and a zero temperature coefficient. Moreover, the high thermal conductivity, as well as the high elastic constant of diamond, provides ultra-high-power handling capabilities [8]. These characteristics have been exploited commercially for high frequency filters and resonators for optical and wireless communications over frequencies ranging from 2.0 to 4.0 GHz [9–14]. Recently, this type of resonator has been used to construct excellent VCSO (Voltage Controlled SAW Oscillator) [15,16] and PLL (Phase Locked Loop) modules [17]. In the area of frequency device technology, Film Bulk Acoustic Resonator (FBAR) devices made from conventional materials have been receiving much attention. This device utilizes the bulk oscillation of a piezo-electric thin film associated with electrodes fabricated on a diaphragm structure [18–20] or a solidly mounted resonator (SMR) [21]. Because of the advantage of high frequency operation made possible by making the film thin, these papers opened a major new research stream in the high frequency device field. However, it has not been introduced to real products for long time, due to the stringent requirement for a highly uniform film deposition technique, resulting in a high cost and low level of performance compared with SAW devices. In 1999, a ⁎ Corresponding author. Tel.: +81 29 861 2770; fax: +81 29 861 2773. E-mail address: [email protected] (S. Shikata). 0925-9635/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.09.006

sophisticated device broke through this barrier through the use of MEMS fabrication technology; Agilent technology [22] introduced a high performance 5 GHz duplexer for mobile communication systems, and many such devices have been use in high frequency wireless systems. A remaining issue for a FBAR/SMR device is that of power handling capability; appreciable damage or irreversible changes are observed at or above +36 dB m power inputs, whereas the units survived the tests at the +29 dB m specification [23]. Based on an analogy with the high power capability of SAW devices, a diamond-based material is a good prospect for overcoming this high power durability disadvantage. In this paper, a new type of frequency device is proposed for the first time that exploits nano-crystalline diamond as the thin film of a FBAR device. A new fabrication process was developed using Si MEMS-based techniques and a successful high frequency FBAR device was constructed. 2. Experimental First, nano-crystalline diamond (NCD) is deposited on a 4 in. diameter silicon wafer by a hot-filament CVD method (SP3 Diamond Technology's reactor model 600). This model combines a proven hotfilament thermal reactor technology with advanced controls to produce nano-crystalline and micro- or poly-crystalline diamond films over a maximum square area of 380 mm on a wide variety of substrate materials, such as carbide-based cutting tools, wear surfaces, Si wafers, etc. Prior to the growth of the NCD, the Si wafers were seeded by a nano-diamond particle ultrasonic treatment. The substrates were heated by the filament radiation from 750 to 850 °C and relatively high CH4 concentration in hydrogen was used for the growth. A schematic of the fabrication process flow of the FBAR device is shown in Fig. 1. First, the 1st level electrode was fabricated on nanocrystalline diamond (NCD). Specifically, Ti (20 nm) / Pt (45 nm) was deposited and patterned by a conventional lithography and lift-off

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Fig. 1. Schematic of the fabrication process flow of a FBAR device.

Fig. 2. Raman spectrum of the nano-crystalline film.

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fabrication run of a FBAR device, ZnO was employed because of the mature technology for its c-axis oriented deposition that has been established for diamond SAW technology. A 300 nm thick layer of Li doped ZnO was deposited on the NCD partly covered with Ti/Pt. Then, in the third step, a 2nd level electrode of Ti (20 nm) / Pt (45 nm) was deposited and patterned using a lithography and lift-off process. Using the 2nd level electrode as the etching mask, the ZnO was removed by wet etching using diluted HCl. The final fabrication step is the deep etching of backside Si using the so-called BOSCH process [24] that has been established in Si MEMS technology. For the patterning of this layer, a double-side mask aligner was used. The FBAR device is designed as one port resonator. 3. Results and discussions

Fig. 3. Photograph of the 4 in. wafer after the FBAR fabrication process.

Fig. 4. Optical microscope photograph of the oscillating section of the FBAR taken from the backside of the wafer.

process. In this process an alignment mark is also fabricated at the same time from Ti/Pt. This is important not only for the alignment of the 2nd level electrode, but also for the back surface lithography for Si etching. Second, the piezo-electric thin film was deposited. In this first

Visible Raman spectrum of the NCD film is shown in Fig. 2. The spectrum consists of sharp features and bands near 1130, 1334, 1350, 1460 and 1580 cm− 1 and have all the characteristics of NCD [25,26]. Appearance of relatively sharp feature at 1334 cm− 1 is an unambiguous signature of crystalline cubic diamond. The features near 1130 and 1460 cm− 1 indicate the presence of nano-crystalline phase of diamond and/or disordered sp3-carbon. The bands near 1350 and 1580 cm− 1 are popularly known as D and G bands, respectively, which are related with graphitic islands. A photograph of a fabricated 4 in. wafer from this first trial is shown in Fig. 3; the wafer contains 400 devices successfully fabricated over the full wafer. The four marks on the wafer edges are the alignment marks used to align multi layers, especially in the final lithography used in patterning the back side of the Si etching. It is remarkable that the fabrication process itself and its process margin is much higher than that of a SAW device made in the beginning of the research that suffered many difficulties. In Fig. 4a, an optical microscope image of the oscillating section of the FBAR taken from the backside of the wafer is shown, while Fig. 4b is a cross sectional schematic. As can be seen from a), the NCD film exhibits a fringe like pattern originating in the strain produced in a free-standing 200 nm film. Also, a several pits can be seen over the area of the film, resulting from residual Si that could not be removed by the BOSCH process in this first trial run. However, they do not appear to influence the device characteristics in initial tests. Fig. 5 a) shows a top view optical microscope photograph of the FBAR device with the design pattern in b). This image shows that we have achieved a fine level fabrication

Fig. 5. (a) Top view optical microscope photograph of the FBAR device with (b) the design pattern.

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Fig. 6. Measured frequency response of a FBAR resonator.

processing of an FBAR device for the first time. For the device testing, the 1st electrode is overlaid by 2nd electrode for the pad area. In order to avoid generating a spurious wave, the resonating area is designed as parallelogram shape with dimensions of 100 µm × 100 µm. The one port resonators fabricated in this way had their frequency responses measured by direct on wafer probing with RF probes using a network analyzer. The typical frequency response of a FBAR device is shown in Fig. 6, as the impedance and phase responses. As can be seen from the figure, the peak in the frequency response was observed near 3.5 GHz. It is important to point out that over the 4 in. wafer, most of the devices have very similar characteristics and more than 90% of them are alive; devices of this structure seem to have the advantage of a high margin using this the fabrication process. As far as the device characteristics are concerned, the response has very high impedance due to the high resistivity of the 1st and 2nd electrodes and it is hard to identify the resonance point. In order to eliminate the effect of the highly resistive electrode, we carried out simulations using the equivalent circuit shown in Fig. 7. By introducing a virtual −80 Ω resistor to reduce R, the low impedance frequency response result is obtained shown in Fig. 8. It can be seen from this figure that the response is more obvious with reduced impedance and the resonance point is clearly observed at 3.494 GHz. The anti-resonating point was 3.560 GHz, as derived from the conventional following equations. fs ¼ 1=2πðL1 C1 Þ1=2 fp ¼ 1=2πðL1 C1 C0 =ðC1 þ C0 ÞÞ1=2 As indicated in Fig. 8, these frequencies are almost the same as the 2 minimum and maximum impedance points. Additionally, the Keff

Fig. 8. Simulated frequency response of a FBAR resonator with an attached virtual resistor of 80 Ω.

value was calculated to be 0.038. This small electro-mechanical coupling coefficient and resulting weak response are due to acoustic impedance of the design. Because of the rigid nature of the 200 nm thick nano-crystalline diamond thin film utilized in the FBAR type structure used for this first trial, coupled with the use of thick and heavy Ti and Pt electrodes, the resonance effect was weak. One plan to increase the response is to optimize the structure, in particular the NCD film thickness, and to select metals with a lower resistance. Another possibility is to utilize an acoustic impedance design that is used for a solidly mounted type of resonator; that is, to avoid the leakage of acoustic energy by introducing low acoustic impedance layers consists of multi-films on NCD. 4. Conclusion A new type of frequency device has been proposed and fabricated for the first time by using nano-crystalline diamond as the thin film of a Film Bulk Acoustic Resonator (FBAR) device. The fabrication process is applied on NCD on a 4 in. Si wafer, including two layers of electrodes, the piezo-electric thin film deposition and a final Si back etching process. The last process was successfully fabricated by using a Si MEMS technique to realize a NCD diaphragm one port resonator. The first-run device showed a high frequency response near 3.5 GHz. The location of the resonance was point was pinpointed to be 3.494 GHz by a simulation that subtracted the effect of a highly resistive electrode, while the anti-resonance point occurred at 3.560 GHz with an electro-mechanical coefficient of 0.038. These relatively low resonance characteristics are due to the acoustic impedance of the design of the device. This new type of thin film bulk resonator opens up new applications in frequency devices modeled on diamond SAW devices, and also in MEMS-based integrated devices. References

Fig. 7. Equivalent circuit used to analyze the resistor effect.

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